Cell surface molecules that are differentially expressed on particular subpopulations of cells may act to regulate cell growth and differentiation or mediate cell interactions with the external environment. Transformed cells may show abnormal expression of particular cell surface molecules, and, in some cases, this abnormal expression may mediate the transformed state. The objective of this project is to define genes that control the cell surface expression of the Thy-1 and Lyt-2 glycoproteins and to determine their mechanism of action. Specific somatic cell mutants provide a way to study the control of the expression of cell surface molecules, particularly when coupled with the use of molecular genetic techniques to introduce specific genes into mutant cell lines. A Thy-1- mutant that acts in trans position to extinguish expression of Thy-1 in hybrids of the mutant with wild-type Thy-1+ cell lines has been described. Nuclear runoff transcription assays will be used to determine whether the gene product defined by this mutant acts transcriptionally. Proteins that bind to the DNA of this Thy-1- mutant and its Thy-1+ revertant will be characterized. Transfection and somatic cell fusion will be used to define regions of the Thy-1 gene that interact with the gene product defined by this mutant. Retroviral insertional mutagenesis will be used to attempt to identify and isolate the gene defined by the mutant. Somatic cell mutants will also be used to define regions important in controlling the expression of the Lyt-2 gene. Southern blotting analysis of one mutant will be used to define a sequence that acts in cis position to activate Lyt-2 expression The gene(s) that act to regulate Lyt-2 and Thy-1 expression in interlineage hybrids will be characterized using a combination of segregation analysis and the introduction of genes into hybrids by transfection.